This invention relates in general to electrostatography and, more specifically, to a flexible electrostatographic imaging member having an improved anti-curl backing layer.
In the art of xerography, a xerographic plate comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged in the dark with a uniform negative electrostatic charge and the conductive layer is utilized as an electrode. The photoconductive layer is capable of photogenerating holes and injecting the photogenerated holes into the contiguous charge transport layer. The charge transport layer in this embodiment must be capable of supporting the injection of photogenerated holes from the photoconductive layer and transporting the holes through the charge transport layer. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electron/hole pairs and injecting the photogenerated holes into the charge transport layer when the imaging member surface is uniformly charged with a positive charge while the conductive layer beneath serves as a negative electrode to receive the injecting holes. The charge transport layer in this embodiment, again, is capable of supporting the injection of photogenerated holes from the photoconductive layer and transporting the holes through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirely. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles.
When one or more photoconductive layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. This is due to the difference in thermal expansion of the substrate and the photoconductive layers and the specific nature of the polymers used for each layer. Curling is undesirable because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, developer applicators and the like during the electrophotographic imaging process thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. Coatings may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the tendency to curl. However, difficulties have been encountered with these anti-curl coatings. Anti-curl layers will also occasionally delaminate after welding into a belt or image cycling in copiers, duplicators, printers and facsimile machines. Delamination is particularly troublesome in high speed automatic copiers, duplicators and printers which require extended cycling of the photoreceptor belt. For example, delamination has occurred in as few as 8,000 cycles. Moreover, delamination is accelerated when the belts are cycled around small diameter rollers and rods.
Since the anti-curl coating is an outermost exposed layer, it has further been found that during cycling of the photoconductive imaging member in electrophotographic imaging systems, the relatively rapid wearing away of the anti-curl coating also results in the curling of the photoconductive imaging member. In some tests, the anti-curl coating was completely removed in one hundred fifty thousand to two hundred thousand cycles. This erosion problem is even more pronounced when photoconductive imaging members in the form of webs or belts are supported in part by stationary guide surfaces, e.g. backer bars) which causes the anti-curl layer to wear away very rapidly and produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, the debris from bisphenol A type polycarbonate anti-curl backing layers tend to deposit on backer bars and other support members. These deposits result in a loud high pitched humming sound emitted during image cycling.
It has also been observed that when conventional belt photoreceptors using a bisphenol A polycarbonate anti-curl backing layer are extensively cycled in precision electrostatographic imaging machines undesirable defect print marks are formed on copies as a result of localized cumulative deposition of anti-curl layer debris onto the backer bars which forces the photoreceptor upwardly and interferes with the toner image development process. Because the anti-curl layer wear causes debris accumulation on the backer bars to gradually increase the dynamic contact friction between these two interacting surfaces, the duty cycle of the driving motor is gradually increased to a point where the motor eventually stalls and belt cycling prematurely ceases. Further, conventional flexible photoreceptor belts carrying an anti-curl layer comprising polycarbonate A on a flexible substrate encounter delamination from bar codes on a substrate during extended image cycling. Moreover, photoreceptors using a bisphenol A polycarbonate anti-curl backing layer normally call for toxic solvents such as methylene chloride to form good coatings. Handling of toxic solvents requires the use of complex, bulky and expensive solvent recovery equipment.